In this paper, a graphene-coating on glass fiber weave has been evaluated as a multifunctional sensingmaterial based on previous work performed by the authors on embedded vertically aligned carbonnanotube forests. First, the graphene-coating has been assessed as a resistive cure-monitoring sensorwhen embedded in thermosetting glass fiber/epoxy laminate to monitor its production. The graphene-coating showed similar results to the resistive cure monitoring of carbon nanotubes. However, thegraphene-coating diverges in its resistive signature upon reaching cure temperature, showing acontinuous resistance decrease in this phase, suggesting a sensitivity to cure-induced shrinkage of theepoxy, not seen in the carbon nanotube sensor. Later, in the cured state, the embedded graphene-coatingfunctions as an excellent temperature sensor, possessing a negative thermoresistive effect. However, asa strain sensor, the graphene-coating does not perform as well as the embedded carbon nanotube sensor,possessing an initial drift in resistance upon its first load cycle and additional drift during constant strainconditions, and when unloaded.

Next-generation aerospace composite structures are expected to evolve from purely mechanical systems into multifunctional structures by integrating additional functionalities through the embedment offunctional filler materials. One promising approach is the incorporation of carbon nanotubes (CNTs) to add sensing capabilities. In this paper, two CNT-based sensing structures are evaluated for transverse pressure sensing up to 20 MPa when embedded in an aerospace-grade glass fibre/epoxy laminate. In pursuit of higher sensing sensitivity, a shift from direct current (DC) to alternating current (AC) based sensing is implemented, enabling the exploration of frequency-dependent sensing behaviour. With this transition, a characterisation and measurement procedure is presented and justified, determining resistive and polarisation effects present in the sensing configurations and evaluating their susceptibility to stray capacitance prior to pressure sensing. The first sensing structure, using embedded Vertically Aligned CNT(VACNT) forests, exhibits pressure sensitivity with the resistive sensitivity increasing at frequencies above the critical frequency of the system, justifying the shift from DC to AC. The reactance shows similar pressure sensitivity except for in a region near 1 MHz, where it becomes pressure insensitive. The second sensing structure, consisting of two embedded VACNT forests separated by a Kapton film, emulates a capacitor. Itsimpedance shows a Kapton-dominated frequency range and a CNT-dominated range, with a transition region in between. Consequently, the pressure response becomes frequency-dependent, as the two constituents not only dominate different frequency ranges but also exhibit different sensitivities to pressure.

In this paper, two strategies to isolate resistive vertically aligned carbon nanotube (VACNT) forestsfrom the conductive carbon fibre environment are presented, enabling embedded sensing with resistivecarbon nanotube sensors in carbon fibre laminates. VACNT forests are used due to their already proventemperature and strain sensing capabilities and ease of placement in prepregs, enabling localisedsensing. To achieve this, a non-permeable separator and a permeable separator are used and compared.Performing cure-monitoring on the VACNT forests during the sample manufacture, it can be concludedthat short-circuits of the resistive sensor are avoided. After manufacture, the temperature and strainsensing capabilities of the VACNT forests when using the two isolation strategies are evaluated. Fromthese measurements, differences in temperature sensing range and sensitivity to strain are observed.

In this paper, Vertically Aligned Carbon Nanotube (VACNT) forests are embedded into two different glass fibre/epoxy composite systems to study their sensing abilities to strain and temperature. Through a bottom-up approach, performing studies of the VACNT forest and its individual carbon nanotubes on the nano-, micro-, and mesoscale, the observed thermoresistive effect is determined to be due to fluctuation-assisted tunnelling, and the linear piezoresistive effect due to the intrinsic piezoresistivity of individual carbon nanotubes. The VACNT forests offer great freedom of placement into the structure and reproducibility of sensing sensitivity in both composite systems, independent of conductivity and volume fraction, producing a robust sensor to strain and temperature.

The next generation of composite structures within aerospace is envisioned to evolve from a strictly mechanical to a multifunctional structure, adding functionalities to the structure by embedment of functional filler material or incorporation of foreign structures. The introduction of carbon nanotubes (CNTs) into the composite structure to achieve sensing capabilities is one example. In this paper, online cure monitoring of aerospace-grade glass fibre/epoxy prepreg laminates is performed by in-situ resistive measurements on embedded vertically aligned carbon nanotube (VACNT) forests. The measured resistance over the course of the cure cycle has a reproducibility in its shape, itself a reflection of the state of the embedded CNTs. The measured resistance is interpreted after studying the morphology of the VACNT forest, cure kinetics and viscosity of the resin, and volumetric changes of both resin and laminate during the cure cycle. The resistive signal is determined to detect the transition between the air-evacuation and consolidation regimes of the laminate compaction and the gel point of the epoxy. Unique observations after the gel point are recorded, theorised to be caused by the build-up of residual stresses in the laminate. The proposed cure monitoring sensor system offers great flexibility, being able to monitor the curing process locally anywhere in the laminate. Additionally, the proposed sensor offers a life-span multifunctionality to the produced component, possessing strain and temperature sensing capabilities in the cured state ideal for structural health monitoring.